Permanent magnetic electron lens system



April 4, 1950 J. H. Rl-:lsNER 2,503,173

PERMANENT MAGNETIC ELECTRON LENS SYSTEM A ngz.

Gtforneg PERMANENT MAGNETIC ELECTRQN LENS SYSTEM Filed oct. V18, 194e 2 sheets-sheet 2 n j l /////,|mvv l/y J7 /16 f7 :inventor j@ ff. ./a'iz @elli/zer E @fw Patented Apr. 4, 1950 PERMANENT MAGNETIC ELECTRON LENS SYSTEM .Johnl Resner, .HaddoneliL N. J., assignorto Radio Corporation of. America, a corporation of Delaware Application October 18, 1946, Serial No. 704,089

(Cl. Z50-49.5.)

12 Claims. l

Thisinvention relates to. improvements in ap- -paratus for focusinga stream of .electrons and,

although not limited to use in apparatus designed forv any one purpose, maybe advantageously usedas a component ,iny an electron microscope.

A means for focusing .a stream of Yelectrons is known as an. electron lens. Its function is analogous, tothe vfunction ,of av lens used for focusinga beam of light. YElectron lenseshave generally been of two distinct types, electrostatic and magnetic, and the latter class may, in turn, be divided into two divisions, namely, the electromagnetic and the permanent magnetic. The

present invention coversimprovements in lenses H of the magnetic type, which may be either permanent or electromagnetic, although in this particular instance the `former isy preferred.

One object of the presenty invention is to provide an improved system of permanent magnetic electron lenses having greatly increased magnification power with good resolution. A

Another object is to provide a permanent magnetic electron lens system utilizing the external magnetic iiux more efliciently.

Another object is to providea permanent magnetic electron lens system utilizing a magnetically permeable shield which at the same time lserves as part ofthe magnetic circuit -of the lens system.

Another object is to provide an improved rpermanent magnetic lens system having-electromagnetic iocusing means.

Another object isv tor provide an improved electron lens system in which either electromagnetic or permanent magnetic lenses maybe used interchangeably.

Still another object is to provide. an 'electron optical lens lsystem to which easyzaccesspmay ,be had for changing/thev magnets orinserting ,specimens.

.These and other objects willbe `more-apparent 'and the invention will hey Abetter understood by reference to the following description taken in connection with the drawingsin'which:

Fig. 1 isa cross section view of `one. type of permanent magnetic lens in which the direction of magnetization of the lmagnets isparallel to the axis of the electron beam. v

Fig.. 2 is. a cross section view of a lens'similar to that of. Fig. 1 but having only VVa singleux gap positioned so as to act upon the electron beam. v

Fig. 3 is. aperspective lpartial viewof another -i'fype of permanent magnetelectron lens constructed according to the present invention but having the directionof magnetization-of the magnets perpendicularvto the axis of the lens.

Fig. 4- is a transverse section View of the lens system of Fig. 3i withl the principal flux paths indicated.

Fig. 5 is a cross section view of another permanent magnet len-s system falling within the scope of` the present invention and also having the principal ilux paths indicated.

Fig. 6- is al perspective partial view oi another type of permanentmagnet-lens system having the 'direction' of magnetization of its magnets perpendicularto the lens axis.

Fig. v'7 is a perspective partial view of still another type in-which direction of magnetization is perpendicular to the lens axis.

Fig. 8 is a cross. section View of another modification in which a. conically shaped magnet is used having its direction of magnetization substantially along an element of the cone.

Fig. 9 shows how the lens system of Fig. 3 may have an electromagnetic focusing control appliedto it.

Fig. 10 shows how'the same may be applied to the system of Fig. 5.

Fig. 11 is a diagrammatic view of a typical electron microscope showing the position of the lens system inrelation to the other parts.

Heretofore, electron lenses have been princivpally of two types, electromagnetic and electrorelative simplicity of lconstruction and operation vhave `been tried. :An example; of this type of lens is shown in the patent to. Bamberg, No. 2,369,796,

diameters. s

The present invention provides a distinct improvement in design and a somewhat diierent principle compared tothe lens system shownI in the YRamberg patent. With the improved type of ,lens magnications of the order of 10,000 diameters with excellent resolution may easily be ob- 55 tained. TheselensI systems are so compact that electron microscopes using them may be made quite small and light in Weight and their cost is but a fraction of the best previously designed in strument using electromagnetic lenses.

In the electron lens system of the present invention paths of magnetically permeable material have been provided to lead the external flux which always surrounds a magnet to ux gaps which thereby become more strongly energized and exert a greater bending influence on the streaming electrons passing by.

Ordinarily a cylindrical or other shaped permanent magnet lens has a strong external field which reacts upon electrons before they enter the lens itself and prevents the formation of sharp images. Attempts have been made to eliminate the effect of this held on the electron path by shielding the lens with magnetically permeable material. Such a shield also prevents stray external magnetic fields generated by other forces from influencing the operation of the lens which shielding is absolutely necessary for successful operation. This is effective as a preventive measure but the manner in which these shields have been used heertofore did not allow the use of the external ux as part of the energizing force in the lens. In the present invention, however, the shield becomes a part of the flux path with the result that the uX gap has its field greatly intensied.

One modification yof a lens system constructed according to the present invention is shown in Fig. l. This is a cross section view of a typical system having component magnets energized in a direction parallel to the path of travel of the electron beam being acted upon by the lens system. In the :figure there is shown a cylindrical magnet l having its bases as poles. To one of the bases is attached an inner concentric cylinder 2 of magnetically permeable material having low retentivity which may be soft iron or cold rolled steel. To the opposite base, which is also the opposite pole of the cylindrical magnet is attached an outer concentric shield 3 of similar magnetic- 'ally permeable material. A spacing ring 4 of nonmagnetic material such as brass separates the end pieces of the inner and outer shields.

The magnetic cylinder I may be thought of as having an inner and an outer flux circuit in parallel. Part `of the path of these circuits is through the cylinder itself and when the cylinder is not associated with any other magnetic substance the return path is through the air. Introducing a magnetically permeable substance into the air path concentrates the ux so that substantially all of it travels through the permeable material instead of through the air. This characteristic of magnets and magnetic fields is Well known.

In order to make a double magnetic lens which can be used to focus a beam of electrons it is necessary to establish two ux gaps which will provide a strong field within a small area. In the present case these gaps are formed-by attaching pole pieces to the magnet and shaping these pole pieces such that one of them acts not only'as an external shield but as a path for leading. the outer iiux to one of the flux gaps. The inner cylindrical pole piece leads the inner iiux to the other ilux gap. In this way almost all 'of the field strength of the magnet is utilized and much higher orders of magnication are possible. i'

The two gap system provides two magnetic lenses, one of which may constitute an objective vlens and the other of which becomes a projector lens as-indicated by the letters'O and P, respectively, on Fig. l. With this combination, high magnications are possible.

For some applications a single gap lens is necessary. For this effect, in which only one of the flux gaps is positioned so that it will aiect the focusing of the electrons, it is necessary to move the unused gap away from the electron path. An illustration of this type of lens is shown in the cross section drawing of Fig. 2. This lens has a cylindrical magnet 5, an inner concentric pole piece 6, an outer concentric shield and pole piece 'l and non-magnetic spacer 3. The flux gap at 9 is the only one which influences the focusing of the electrons since the other gap l0 is too far removed to have any eifect.

The magnet does not need to be cylindrical in shape. Any hollow tubular form may be used such as square, polygonal, or oval. The form need not even be hollow tubular. The magnet system may consist of one or more bar magnets and the shield and inner conducting member may assume correspondingly limited forms, such as bars.

In the modification of the invention just described the alternate forms of the component magnets have all had their direction of magnetization substantially parallel to the axis of the path of the electron beam or the optical axis of the lere. It is not necessary, however, that the magnets be arranged in this way. They may be placed such that their direction of magnetization is perpendicular to the optical axis of the lens. One illustration of this type is shown in Fig. 8., This iigure shows a lens system in which two pairs of bar magnets II--IZ and I 3-M are arranged on opposite sides of a magnetically permeable core l5. All of the magnets have their poles in parallel. Connecting the -outer poles of each pair of magnets are magnetically permeable yokes I6 and Il. Spool ends I8 and I9 of magnetically permeable material complete the assembly.

The' principal flux paths through this system are shown in Fig. 4. Here it is shown that the outer shield, made up of the yokes I6 and I1 serves as part of the path of the magnetic circuit conducting the flux to the flux gaps where most of it can be utilized.

Fig. 5 shows a longitudinal section View of another modiiication of the system in which only two bar magnets 20 `and 2| are used. The principal flux paths are indicated.

Other examples of combinations of bar magnets which fall within the scope of the invention are shown in the drawings. Fig. 6 is a partial view of one end of a lens system having eight bar magnets. Only four of the magnets appear in the drawing, the other end of the system being an exact duplicate. In the drawing the magnets `22, 23, 24, and 25 `are each joined to an inner magnetically permeable core 26 and have their opposite poles connected to magnetically permeable yokes 21, 218, 29, and 130, respectively. The yokes are joined to magnetically permeable end pieces, one of which 3| is illustrated.

Fig. 7 shows how 3 bar magnets 32, 33, and 34 may be grouped symmetrically around a core 3l.

The invention is not limited to use of magnets energized either parallel or perpendicularly to the axis of the lens. The magnets may be energized at some other angle such as illustrated in Fig. 8. This figure shows a conically shaped magnet 35 surrounded by a soft iron shield 37 and having soft iron pole pieces 38. In this Icase the shield also serves as part of the path through which the Lble metal.

:ux Vis 5led tozcomplete Ithefmagnetic' circuit across 'the sap.

In the .construction ofzallfof Vthese `lenses the magnets may be made'or any -stronglyrmagnetizaalloyscontaining iron, aluminum,tnickel,and cohalt butotherrmagnetized ymaterial maybe usedi strength4 and stability.' are not primary vrequirements l Whenr bar magnetscare .used they needtnot be Vof rectangular Across section since '.:otheinshapes .such as circular` or-.wedge shape will serve vequally Well.

The `modification in which bar .magnets are .used is perhapsfmost preferable lfrom a practical standpoint. In this type it is possibletorvary the maximum strength overV azwide range, by using different numbers of energizing units. .Another advantage is that iocusingmaybe changed by changing the position'. ofr an Vauxiliarymagnet or a shorting bar. vPole piece'gapsandco'lumn spaces areaccessible for external cooling. ortem- :perature control `which vlatter factor is desirable to obtain constancy of gain or resolution. Image rotation lcan .also rbe reduced to a "negligible-f amount.

The system which has been described isnot limited to the'one or two gaptypes whichhave been illustrated. .Any'number of .units maybe `assembled together to energize anydesired .number ofv gaps.

ways. Oneway )isto vary theuaccelerating potentlal of the electrons. in order to change their velocity. Another way is to placeauxiliarycoils in the magnet system and vary the current through them. Figure 9 illustrates-how this focusing :system may be applied to the lens system `of Fig. 3. Two coilsf39 and 4D arefwound on part of the outer yokes IGand Il, respectively,

' and areenergized bya variable .source of direct current. Figure 10 shows how a single coil 4| may be Wound on the inner core of the system illustrated in Fig. 5.

Any of the lens systems described in the preceding paragraphs may be used in any device in which it is desired to focus a moving stream of electrons. They have particular application in electron microscopes and diffraction cameras. An example of a typical use is shown diagrammatically in Fig. 11. This flgure shows the principal elements of an electron microscope including a source of emitted electrons 42 which may be a specially constructed cathode, an aperture 43, a specimen or object 44 to be examined, a lens system 45, a fluorescent screen 46 and an enclosing evacuated chamber 41 having a viewing window 48. The cathode emits a stream of electrons which strike the specimen and at any point being examined'these electrons diluse in a regular It is 'preferred tonseoneof the Alm'co y This may be done in either one of two gf.;

pattern. This diffused stream pattern then enlv `havebeen described. On soft .ironcorcsfof-.suitable size, `i coils :are v'wonndzand'each coil is energized so as to giveit the same .polarity zas-rit Ywould have V-had vhad the magnet ybeen a A.permanent one.

There has thus been described :an improve :electron lens "system having, fundamenta1ly,two

gaps, one of which is energized .byA means of flux Jconducted to 'the fgap through ashielding struc- .ture

Thus the magnetically permeable `material which acts .as'theshield also adds to the strength of. thelens because of. the manner in which it is '/used. Themagn'ets may be either of the ypermanent-orelectro type and yelectrical focusing .means ymay be included 'in the system.

.1I claim as my invention: .1.. An electron lens system comprising magnetic means having associated therewith andconnected -thereto 'structure constituted of ya -plurality of members of magnetically permeable materialhaving lowiretentiv'rty, at least twoi relatively narrow lilux Vgapswithin said `lensV system, said kdifferent members'. of .said Apermeable structurev beingl con- :.nectedv to opposite poles of said magnetic means Aand Vserving .as vpart I of the magnetic circuits fthroughthe.uxfgaps and atleast one -of the members of said permeable structure surrounding v'said'magneticmeans .and servingfas ashield for .the lens system.

2. An electron lens system. comprising'relative- `ly stronglyv magnetized magnetic means having at least two flux gaps .associated therewith, and

.shielding means surrounding .said magnetic 'means said shieldingmeans beingconstituted of .magnetically .permeable` material with low reten- 'tivity .connected to .one pole `of said magnetic A.means randshapedf such that one of said .gaps

is energized Vby magnetic linx vconducted .from

'.'sai'd magnetic means'and `.traversing said shielding means. y

.3. AAn electronlens systemcomprising relatively :strongly magnetizedmagneticmeans having associated 'therewith andfconnected-thereto Vstructure .'.constituted of magnetically permeable `material having low retentivity, said structure forming at least two flux gaps energized by said magnetic means, and shielding means surrounding said magnetic means and forming part of said magnetically permeable structure shaped such that at least one of said gaps is energized by magnetic flux conducted from said magnetic means and traversing said shielding means.

4. In an apparatus including means for focusing a. beam of electrons, an electron lens system comprising a permanent magnet system having its components spaced around a portion of the path of said beam, and having at least two ux gaps, magnetically permeable shielding means connected to and surrounding said magnetic components, said shielding means forming at least part of the path by means of which at least part of the external flux of said magnet components is led to said ilux gaps.

5. In an apparatus for focusing a beam of emitted electrons, an electron lens system comprising a permanent magnet system having magnetic means with its direction of magnetization substantially parallel to the axis of the lens system, and with its component parts arranged axially about said electron beam, said magnet system having an inner and an outer flux path and having two gaps provided by attached pole pieces, one of said gaps being energized by flux traversing said inner path and the other of said gaps being energized by flux traversing said outer path with at least one of said gaps being positioned so that its eld exerts a focusing effect on the electrons in said beam.

6. An electron lens system comprising a hollow tubular permanent magnet having bases as poles and having an inner and an outer ux path,

said magnet having two gaps provided by attached pole pieces shaped such that one of said gaps is energized by flux traversing'said inner path while the other of said gaps is energized by flux traversing said outer path.

7. An electron lens system comprising a hollow l tabular permanent magnet having bases as poles and having an inner and outer flux path, said magnet having two gaps provided by separate magnetically permeable structures attached to opposite poles of said magnet, the rst of said structures serving as a path for said inner ux "i and the second oi said structures serving asa path for said outer ux, whereby the rst of said gaps is energized by the ux traversing said inner path and the second of said gaps is energzed by the flux traversing said outer path.

8. A n electron lens system comprising a hol low tubular permanent magnet having bases as.

poles, an inner concentric shell of magnetically permeable material attached to one of said poles and forming a ux gap adjacent the other of said poles, and an outer concentric shell of magnetie cally permeable material attached to said second mentioned pole and forming a ux gap adjacent said iirst mentioned pole,

9. In an apparatus for forming a beam of I emitted electrons, an electron lens system comprising a hollow tubular magnet having bases as poles and having an inner and an outer flux path, said magnet having two gaps provided by c separate magnetically permeable members attached to opposite poles of said magnets, one of said members serving as the path of said inner ux and the other of said members serving as the path of said outer flux, whereby the iirst of said gaps is energized by the lux traversing said inner path and the second of said gaps is energized bythe ux traversing said outer path and at least one of said gaps being positioned `so that its eld inuences the direction of travel of the electrons in said beam.

10. In apparatus for focusing a beam of emitted electrons, an electron lens system comprising a magnetic system having its component magnets arranged axially around a portion of the path of said beam, separate magnetically permeable means connected to the opposite poles of said component magnets for conducting substantially all of the external ilux to ux gaps positioned at predetermined points, saidv gaps being so placed that at least one of them influences the direction of travel of the electrons in said beam.

11. An electron lens system comprising magnetic means having its direction of magnetization perpendicular to the lens axis, at least two iiux gaps within said lens system, separate magnetically permeable members of low retentivity connected to opposite poles of said magnetic means and serving as part of the magnetic circuits through the iiux gaps.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,212,206 Holst et al. Aug. 20, 1940 2,305,761 Berries et al. Dec. 22, 1942 2,369,796 Ramberg Feb'. 20, 1945 

